In this paper, we present a modified nanosphere lithographic scheme that is based on the self-assembly and electroforming techniques. The scheme was demonstrated to fabricate a nickel template of ordered nanobowl arrays together with a nickel nanostructure array-patterned glass substrate. The hemispherical nanobowls exhibit uniform sizes and smooth interior surfaces, and the shallow nanobowls with a flat bottom on the glass substrate are interconnected as a net structure with uniform thickness. A multiphysics model based on the level set method (LSM) was built up to understand this fabricating process by tracking the interface between the growing nickel and the electrolyte. The fabricated nickel nanobowl template can be used as a mold of long lifetime in soft lithography due to the high strength of nickel. The nanostructure&ndash;patterned glass substrate can be used in optical and magnetic devices due to their shape effects. This fabrication scheme can also be extended to a wide range of metals and alloys.

Figure 7: SEM image of a FIB cross-section on the edge of PS template after nickel electroforming.

Mentions:
In this study, the multiphysics model based on level set method (LSM) was built up to investigate this metal forming process through tracking the position of the metal and electrolyte interface. The LSM has already been shown to be a powerful method for studying deposition processes and tracking the evolution of the moving interface between deposits and electrolyte [25]. In the LSM, a scalar variable, φ, is defined over the entire computation region, and the set of locations φ = 0.5 in our model (usually, the zero level set φ = 0 is used) defines the position of the interface. Figure 6 shows that the nickel deposits on the top of the nanospheres have merged, and the interstitial between the nanospheres are blocked at T = 0.5 s. The fast electroforming rate is due to that a high current density (500 A/m2) was used in the simulation to decrease the computation time. However, a low current density (15 A/m2) was used in the experiment to avoid the hydrogen bubbles and black deposits. The simulation also reveals that nickel deposits can fill the whole interstitials among the nanospheres once the top openings are not blocked by growing metal; in other words, PS nanopsheres are not closely packed together (simulation results are not shown here). The FIB milled cross-section (Figure 7) shows the complete filling of nickel deposits around the nanospheres on the edge of the PS nanotemplate where multiple layers of PS nanopsheres are self-assembled. Undoubtedly, a porous nickel nanostructural material can be made in this manner by using the close-packed three dimensional nanospheres as template for electroforming.

Figure 7: SEM image of a FIB cross-section on the edge of PS template after nickel electroforming.

Mentions:
In this study, the multiphysics model based on level set method (LSM) was built up to investigate this metal forming process through tracking the position of the metal and electrolyte interface. The LSM has already been shown to be a powerful method for studying deposition processes and tracking the evolution of the moving interface between deposits and electrolyte [25]. In the LSM, a scalar variable, φ, is defined over the entire computation region, and the set of locations φ = 0.5 in our model (usually, the zero level set φ = 0 is used) defines the position of the interface. Figure 6 shows that the nickel deposits on the top of the nanospheres have merged, and the interstitial between the nanospheres are blocked at T = 0.5 s. The fast electroforming rate is due to that a high current density (500 A/m2) was used in the simulation to decrease the computation time. However, a low current density (15 A/m2) was used in the experiment to avoid the hydrogen bubbles and black deposits. The simulation also reveals that nickel deposits can fill the whole interstitials among the nanospheres once the top openings are not blocked by growing metal; in other words, PS nanopsheres are not closely packed together (simulation results are not shown here). The FIB milled cross-section (Figure 7) shows the complete filling of nickel deposits around the nanospheres on the edge of the PS nanotemplate where multiple layers of PS nanopsheres are self-assembled. Undoubtedly, a porous nickel nanostructural material can be made in this manner by using the close-packed three dimensional nanospheres as template for electroforming.

In this paper, we present a modified nanosphere lithographic scheme that is based on the self-assembly and electroforming techniques. The scheme was demonstrated to fabricate a nickel template of ordered nanobowl arrays together with a nickel nanostructure array-patterned glass substrate. The hemispherical nanobowls exhibit uniform sizes and smooth interior surfaces, and the shallow nanobowls with a flat bottom on the glass substrate are interconnected as a net structure with uniform thickness. A multiphysics model based on the level set method (LSM) was built up to understand this fabricating process by tracking the interface between the growing nickel and the electrolyte. The fabricated nickel nanobowl template can be used as a mold of long lifetime in soft lithography due to the high strength of nickel. The nanostructure&ndash;patterned glass substrate can be used in optical and magnetic devices due to their shape effects. This fabrication scheme can also be extended to a wide range of metals and alloys.